This invention relates to stop circuits for electric arc furnaces.
Furnace control systems for electric arc furnaces have many functions, two of the most important being to control the supply of electrical energy to the carbon electrodes, and to control the mechanisms for raising, lowering and stopping the electrodes.
In the typical operation of an electric arc furnace, it is quite normal to interrupt the arcing process several times during a cycle of operation of the furnace, to enable various testing, and sampling operations to be carried out, and for observation purposes. If the operator simply opens the switch in the primary of the power transformer for the electrodes, this step, with a full current load to the electrodes, puts mechanical stresses on the transformer, and also tends to create voltage spikes, which tends to break down insulation, and there are other disadvantages known to those skilled in the art.
The operator is instructed to watch the kiloamp meters (which measure the current to the three electrodes) as the electrodes are raised, and when the current drops to a desired safe level, to open the power switch in the primary of the power transformer. This achieves minimum damage to the transformer and associated equipment.
However, operators are under pressure to perform a number of jobs, and to perform them fast, and in the press of work, occasionally, or sometimes rather often, will simply open the power switch, under full current load, and thereafter raise the electrodes, because this takes less time than trying to monitor the operation, and it frees the operator to do other jobs.
The present invention overcomes the above problems by providing a controlled stop circuitry which frees the operator for carrying out other duties at cycle-interrupt time, by being automatically operable, when once activated, to partially raise the electrodes and to open the transformer switch, but only after the electrode current has decreased to a safe value.
More particularly, the circuitry, when actuated in a designated manner, interrupts an operating cycle by raising the electrodes, while simultaneously sensing the current supplied to them, and is effective, when the current decreases to a safe level, to automatically open the transformer switch and stop movement of the electrodes in partially raised positions.
The circuitry also permits, as an option to the above procedure, the carrying out of the above steps, except that it delays opening of the transformer switch until the electrodes reach the top of their travel.
The present invention further contemplates a controlled stop circuitry so designed that it can be readily patched into an existing furnace control system and make use of certain of the systems components in carrying out the circuitry's functions.
The inventive circuitry is also so designed that it can be patched into the furnace control system in such a manner as to give the operator the option of using the system in its usual fashion, or of using the controlled stop circuitry in its intended fashion.
The invention will be explained in connection with the accompanying drawings, wherein the single FIGURE is a schematic drawing of a system embodying the concepts of the present invention.
The controlled stop circuitry of the present invention has been incorporated in, and is a modification of, the furnace control system of a furnace owned by Cascade Steel Rolling Mills, Inc. of McMinnville, Oregon. This system was purchased from Lectromelt of Pittsburgh, Pennsylvania, under the brand name "Lectrostat Electrode Positioning Control System." The automatic electrode regulating portion of the system was produced by Wer Industrial of Grand Island, New York.